One of the earliest known procedures for producing shell molds is the Kroning process, which involved the use of a sand having coated thereon a dry solid resin, usually a phenol-formaldehyde novolac resin, catalyzed with hexamethylene tetramine. This catalyzed resin binder is stable under ordinary atmospheric ambient conditions but is catalytically polymerized, that is, cured, at elevated temperatures. In the practice of the Kroning method, the dry coated catalyzed sand, usually AFS 80-90 grain fineness, for example, is packed into a metal pattern capable of being heated. The pattern is pre-heated to a sufficiently high temperature, e.g. 450.degree.-500.degree. F. to achieve the polymerization of the binder on the sand disposed thereagainst. Typically only a thin layer of the sand mixture is cured.
One of the main advantages of manufacturing shell cores and molds is the fact that a relatively thin shaping surface is fabricated, and that the shell core and mold manufacturing procedure does not require the hardening of a quantity of sand which would otherwise have filled the entire region within the shell core or mold. Typically in the traditional "dry" hot box method the pattern is then inverted and uncured sand-binder is dumped and separated from the relatively thin "shell" of cured sand-binder, and the dumped uncured sand-binder portion is salvaged for reuse.
Although the method has enjoyed widespread commercial success, there are inherent disadvantages associated with the use of the heated metal patterns. For example, the patterns are typically very expensive, and are fabricated with great difficulty due the need for compensating for thermal expansion and avoidance of pattern warpage at elevated temperatures.
Another method heretofore available for the manufacture of shell cores and molds involved the use of "wet" or liquid binders such as, for example, liquid furfuryl alcohol resin binders, in so-called "cold box" methods. In a cold box method heretofore available, for example, a relatively thin layer of sand coated with a liquid polymerizable binder is placed against the pattern. This is often done manually and the use of vibrators and the like, to increase interparticulate contact must be done carefully insofar as substantially vertical portions of the packed sand may tend to dislodge or separate.
In the latter methods the patterns which are used are provided with means for passing gas therethrough, and, typically, the entire mass of placed sand is cured by passing a resin-polymerizing catalyst in the gas phase therethrough to achieve curing of all the sand mixture.
The latter so-called cold box methods of making shell molds suffer the disadvantage of requiring hand packing or hand placement of the initially used binder-sand mixture with the resulting inconsistent and non-uniform sand-depth dimensions and oftentime uneven packing.
Illustrative of patents which have related to these areas include U.S. Pat. No. 3,008,205 to H. O. Blaies, Jr. "Shell Type Molds and Cores," U.S. Pat. No. 2,874,428 to J. L. V. Bonney, Jr. "Method of Hardening of Sand Cores and the Like", U.S. Pat. No. 3,145,438 to R. H. Kottke, et al., "Gas Cure of Organic Bonds for Sand and Abrasive Granules", U.S. Pat. No. 3,428,110 to J. Walker, et al., "Process for the Production of Foundry Cores and Molds" and U.S. Pat. No. 3,639,654 to J. Robins, for "Gaseous Halo-Sulfonic Acid Anhydride Catalysts for Curing Furfuryl Alcohol and Furan Resins."
It is an object of the present invention to provide a cold box method of making shell cores and molds, which method does not require the manual placement of a relatively thin layer of sand-binder mixture for complete curing thereof, and which method also provides automatically for the curing of the sand-binder mixture adjacent the pattern wall to a predetermined desired thickness.